When a wind turbine is mounted on a floating structure in a sea area having a deep water depth, a semi-submersible type, a tension leg platform (TLP), or the like may possibly be employed. In such cases, the wind turbine or the like and the floating structure are integrally assembled and subjected to test operation onshore, and then towed to an installation site and moored by a mooring system.
On the other hand, in the case of the spar-type, the spar is a floating structure floating in an upright standing state like a fishing float, and a large part of the float is submerged under the sea level by injection of ballast water, or the like. The spar is relatively unlikely to swing, and can support a heavy upper structure. In the case of a spar-type floating structure equipped with a wind turbine, the draft of the float may be as deep as approximately 40 m to 80 m, and the float portion becomes a large structure of approximately 1000 t (ton) or more.
As the spar-type offshore structure, a float-type fluid-force utilizing system is provided as described in International Publication No. 2013/065826, for example. In this system, a horizontal-axis wind wheel or a vertical-axis wind wheel for utilizing wind force is disposed at an on-water portion thereof, and a horizontal axis water wheel or a vertical axis water wheel for utilizing tidal current force is disposed under the water level, and the system utilizes the horizontal axis water wheel or the vertical axis water wheel as ballast.
When the spar-type floating wind turbine is installed at an offshore installation site, as described in Japanese patent application Kokai publication No. 2012-201219, for example, there is provided a method of constructing a floating wind turbine, including: a carrying step of floating a float, which is a lower structure, in a laid-down state on the sea and towing the float with a towing vessel, or mounting the float on a barge, and carrying the float to an offshore installation site; a float upright providing step of floating the float and then bringing the float into an upright standing state by ballast adjustment, and performing draft adjustment; a float mooring step of installing a deck on the float, and connecting one end of a mooring line to the float and connecting the other end thereof to an anchor provided on the bottom of the sea to stabilize the float; and a tower installing step of hanging down a tower, which has been carried to the offshore installation site separately with a barge or the like, by using a crane vessel, and installing the tower on an upper portion of the float.
In this construction method, in order to perform easy and safe construction on the sea and to secure the stability in strong winds or waves, when the tower, which is to be provided upright on the float, is installed on the upper portion of the float, the swing of the tower is controlled by a mass damper provided to the tower or a hanging jig of a crane for hanging down the tower, and the swing of the float is controlled by a control moment gyro provided inside the float.
When the float is moored, and the tower is hang down to be integrated with the moored float by a crane vessel, the crane vessel, which is expensive for use, is necessary. Moreover, since the tower hung down by the crane is likely to be affected by winds, which requires control of the swings of the tower and the float. Accordingly, there is a problem in that the tower installation construction work is difficult to proceed.
Meanwhile, as described in Japanese patent application Kokai publication No. Hei 10-236385, in order to safely and efficiently load and unload a large-sized cargo such as a container crane by utilizing hull buoyancy caused by loading and discharging ballast water, there is proposed a method of loading and unloading a large cargo, in which ballast water is loaded in or discharged from a carrier vessel including a ballast tank and a pair of arm-shaped structures protruding from an end portion of the hull in a stern direction, and a large cargo is loaded and unloaded by utilizing the hull buoyancy caused by loading and discharging the ballast water (see for example Patent Document 3).